Laser printers for printing images in response to the reception of signals from a microcomputer, such as an IBM PC or the like, are well known. In a typical printer of this type, a memory is provided for storing image signals from the microcomputer. The microcomputer formats the signals in the manner required for the printer, and the printer memory is adapted to store bit mapped image pages of images prior to initiating printing. The printer sequentially reads out from the bit mapped memory to modulate the light beam of a laser diode. The light beam of the laser diode is controlled to scan a photoconductive drum, for example by the use of a rotating polygonal mirror. The image thus stored on the drum is then developed and transferred to paper or the like in a manner conventional to xerographic copying devices.
Typical laser printers of this type are disclosed, for example, in U.S. Pat. Nos. 3,816,652; 4,383,755 and 4,783,680.
Laser printers of this type are generally controlled to provide resolutions of about 300 dpi. Vertical resolution is a function of the rotational rate of the polygon mirror, while horizontal resolution is determined by the control rate of switching of the laser diode.
In the conventional use of a laser printer to print fax images, an accompanying microcomputer is provided with a fax modem, and circuitry adapted to demodulate and decompress the signals. The microcomputer converts the signals to be compatible with the format of the laser printer, and transmits signals in accordance with this format to the printer.
This technique has a number of disadvantages resulting from the different resolutions of the fax images and the normal printer characteristics. Thus, group 3 fax operates at resolutions of approximately 96.times.204 or 196.times.204 lines per inch. Since the resolutions of the fax images are not even submultiples of the resolution of the conventional printer images, interpolations must be provided to match the fax images to the format of the normal printer resolution, and various lines of the image must be repeated. This results in degradation of the printed fax image.
Even greater degradation occurs with half tone images since the spot size is too large. A further disadvantage of the above technique is that the page throughput rate is very slow. The inputting of the fax signals directly into the microcomputer also does not provide a ready source of signals for the screen display of the microcomputer, due to the substantially lower resolution of the screen. In addition, this technique requires the provision of a modem, fax board and analog phone line connected to each microcomputer, and frequently does not permit use of the respective microcomputer during the reception of a fax message.
Consequently, current laser engines are adapted to serve either as a fax printer or a printer for microcomputer signals, but not both.